Letter pubs.acs.org/JPCL
Atomic-Scale Perspective of Ultrafast Charge Transfer at a Dye−Semiconductor Interface Katrin R. Siefermann,†,□ Chaitanya D. Pemmaraju,‡ Stefan Neppl,† Andrey Shavorskiy,§ Amy A. Cordones,∥,● Josh Vura-Weis,∥,△ Daniel S. Slaughter,† Felix P. Sturm,† Fabian Weise,†,▲ Hendrik Bluhm,§ Matthew L. Strader,§,▼ Hana Cho,§ Ming-Fu Lin,†,∥,△ Camila Bacellar,†,∥ Champak Khurmi,†,% Jinghua Guo,# Giacomo Coslovich,⊥ Joseph S. Robinson,⊥,▽ Robert A. Kaindl,⊥ Robert W. Schoenlein,†,⊥ Ali Belkacem,† Daniel M. Neumark,†,∥ Stephen R. Leone,†,∥,○ Dennis Nordlund,◆ Hirohito Ogasawara,◆ Oleg Krupin,▽,¶ Joshua J. Turner,▽ William F. Schlotter,▽ Michael R. Holmes,▽ Marc Messerschmidt,▽,& Michael P. Minitti,▽ Sheraz Gul,#,+ Jin Z. Zhang,+ Nils Huse,■ David Prendergast,‡ and Oliver Gessner*,† †
Ultrafast X-ray Science Laboratory, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡ The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ∥ Department of Chemistry, University of California, Berkeley, California 94720, United States ⊥ Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States # Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ▽ Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States ○ Department of Physics, University of California, Berkeley, California 94720, United States ◆ SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States ¶ European XFEL GmbH, 22761 Hamburg, Germany + Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States ■ Physics Department, University of Hamburg and Max-Planck Institute for Structure and Dynamics of Matter, 22761 Hamburg, Germany S Supporting Information *
ABSTRACT: Understanding interfacial charge-transfer processes on the atomic level is crucial to support the rational design of energy-challenge relevant systems such as solar cells, batteries, and photocatalysts. A femtosecond time-resolved core-level photoelectron spectroscopy study is performed that probes the electronic structure of the interface between ruthenium-based N3 dye molecules and ZnO nanocrystals within the first picosecond after photoexcitation and from the unique perspective of the Ru reporter atom at the center of the dye. A transient chemical shift of the Ru 3d inner-shell photolines by (2.3 ± 0.2) eV to higher binding energies is observed 500 fs after photoexcitation of the dye. The experimental results are interpreted with the aid of ab initio calculations using constrained density functional theory. Strong indications for the formation of an interfacial charge-transfer state are presented, providing direct insight into a transient electronic configuration that may limit the efficiency of photoinduced free charge-carrier generation. SECTION: Spectroscopy, Photochemistry, and Excited States
I
solar cells (DSSCs) are a particularly prominent example of chemically engineered photovoltaic devices that convert sunlight into electricity through the combination of an efficient light absorber
nterfacial charge-transfer dynamics play a crucial role in a number of emerging photoelectrochemical technologies, in particular, for photovoltaic and photocatalytic applications. An atomic level understanding of the transient electronic configurations that govern the conversion efficiencies of photoinduced excitations into separate charges is essential in order to exploit the full potential of novel photon-harvesting schemes. Dye-sensitized © 2014 American Chemical Society
Received: June 20, 2014 Accepted: July 25, 2014 Published: July 25, 2014 2753
dx.doi.org/10.1021/jz501264x | J. Phys. Chem. Lett. 2014, 5, 2753−2759
The Journal of Physical Chemistry Letters
Letter
Figure 1. Two competing models for the sequence of charge-injection at N3 dye−ZnO nanocrystal interfaces after photoexcitation (step 1) of the dye. (A) Two-state injection model in which the electron is temporarily retained in a 3MLCT state (step 2) on the dye molecule. (B) Intermediate interfacial state model in which the electron is temporarily retained in an interfacial complex (IC, step 2) at the dye−semiconductor interface. In both cases, step 3 indicates the release of the electron into the ZnO conduction band (CB) as a free charge carrier.
(dye molecules) with a nanostructured semiconductor substrate.1−4 Dye−semiconductor charge injection mechanisms have been investigated in numerous studies with a particular emphasis on the associated injection rates.5−22 However, owing to the complexity of the interfacial configurations, an unambiguous interpretation of the experimental findings is often challenging, in particular, on a level of detail commensurate with advanced theoretical predictions.3,23−26 Here a femtosecond time-resolved X-ray photoelectron spectroscopy (TRXPS27,28) study is presented that probes the transient interfacial electronic configuration of the ruthenium-based dye N3 (bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II)) attached to a zinc oxide (ZnO) nanoparticle film within the first picosecond after photoexcitation of the dye. The atomic-site-specific TRXPS results in combination with density functional theory (DFT) calculations provide strong indications for a transient charge-transfer state that is established across the N3−ZnO interface within 500 fs. The findings are of direct relevance with respect to a decade-long controversy surrounding the fundamental electronic dynamics that underlie performance differences in TiO2- and ZnO-based DSSCs. A number of studies have demonstrated that the differences between these systems, for example, in the critical photon-tocurrent conversion efficiencies cannot be explained by common system benchmarks such as the dye−semiconductor energy level alignment or the electron mobility of the substrate material.13,29,30 Instead, the atomic-scale electronic coupling and transient interfacial electronic structure are believed to play key roles in determining the device function.9−13,31 Ultrafast time-domain studies in the optical, infrared, and terahertz regimes have identified distinct differences between the dynamic response of dye−ZnO and dye−TiO2 interfaces.5,8−13,17 In particular, transient signals from ZnO-based dye-sensitized systems are marked by prominent “slow” components, which evolve on time scales ranging from a few to hundreds of picoseconds. These response times are orders of magnitude longer than the typical sub-50 fs dynamics of TiO2based interfaces and are, therefore, often associated with ZnO cell performance limitations.5,8−13 While these dramatic differences of time scales are consistently detected in a variety of experiments, their physical origin has long been a matter of debate, leading to two competing models for the underlying mechanisms (Figure 1). In the two-state injection model5−7 (Figure 1A), only a small fraction of the initially excited singlet metal-to-ligand charge-transfer (1MLCT) states directly injects an electron into the conduction
band (CB) of ZnO within
